Red,
or dark meat is made up of muscles with fibers that
are called slow-twitch.
These muscles are used for extended periods of activity,
such as standing or walking, and need a consistent energy
source. The protein myoglobin stores oxygen in muscle
cells, which use oxygen to extract the energy needed for
constant activity. Myoglobin is a richly pigmented protein.
The more myoglobin there is in the cells, the redder, or
darker, the meat.

When
dark meat is cooked, myoglobin's color changes depending
on what the meat's interior temperature is. Rare beef is
cooked to 140° F, and myoglobin's red color remains
unchanged. Above 140° F, myoglobin loses its ability
to bind oxygen, and the iron atom at the center of its molecular
structure loses an electron. This process forms a tan-colored
compound called hemichrome, which gives medium-done
meat its color. When the interior of the meat reaches 170°
F, hemichrome levels rise, and the myoglobin becomes metmyoglobin,
which gives well-done meat its brown-gray shade.

White
meat is made up of muscles with fibers that are called
fast-twitch. Fast-twitch
muscles are used for quick bursts of activity, such as fleeing
from danger. These muscles get energy from glycogen, which
is also stored in the muscles.

White
meat has a translucent "glassy" quality when it is raw.
When it's cooked, the proteins denature and recombine, or
coagulate, and the meat becomes opaque and whitish.

Cows
and pigs are both sources of dark meat, though pig is often
called "the other white meat." Pigs' muscles do contain
myoglobin, but the concentration is not as heavy as it is
in beef. Chickens have a mixture of both dark and white
meat, and fish is mainly white meat.

Different
types of meat require different cooking times. The best
way to determine if meat is done is to use a meat thermometer
to check the internal temperature. Beef can be cooked to
a variety of temperatures: rare (140° F), medium (160°
F), and well-done (170° F). Pork, chicken, and fish
have less leeway. It's recommended that pork be cooked to
170° F, chicken to 180° F, and fish to 165°
F.